This application describes the development of a unique Consortium (the Sterol and Isoprenoid Diseases Research Consortium or STAIR) to study a group of diseases bound by common biochemistry, impact on health, and rarity: Cerebrotendinous Xanthomatosis (CTX), Hyperimmunoglobulinemia D with Periodic Fever Syndrome (HIDS), Niemann-Pick Disease Type C (NPC), Sitosterolemia, Sjogren-Larsson Syndrome (SLS), and Smith-Lemli-Opitz Syndrome (SLOS). STAIR activities will be performed by a team of investigators chosen for their clinical research strengths and resources, a long history of collaboration, a diverse geographic distribution to allow maximal access by potential research subjects, their individual motivation to improve the health of patients, and the commitment of their institution to support the Consortium. In five years, STAIR will conduct two major clinical studies (a longitudinal natural history study of NPC and a therapeutic trial to evaluate the efficacy of antioxidant therapy in SLOS), and six pilot research studies involving patients with either SLS, SLOS, CTX, HIDS or Sitosterolemia. Together with the Intramural NIH program, the Consortium will support a full-scale training program in the field of sterol and isoprenoid diseases and share its resources and data with the NIH RDCRN. Participating institutions include Oregon Health and Science University (OHSU), Eunice Kennedy Shriver National Institute of Child Health and Human Development, Children's Hospital of Pittsburgh of the University of Pittsburgh Medical Center, Cincinnati Children's Hospital Medical Center, University of Nebraska Medical Center, and University of Manitoba (Canada). OHSU will be the administrative home of the Consortium. Patient support organizations (Smith Lemli Opitz/RSH Foundation, Hide and Seek Foundation, Ara Parseghian Medical Research Foundation, Dana's Angels Research Trust, Foundation for Ichthyosis and Related Skin Types, and United Leukodystrophy Foundation) will participate in Consortium activities. In summary, STAIR will foster multidisciplinary clinical research, promote training and education and support projects to explore promising leads in the understanding, diagnostics, and treatment of sterol and isoprenoid diseases.